FIG. 1 shows an example of one known turbine engine 10 having a compressor section 12, a combustor section 14 and a turbine section 16. In the turbine section 16, there are alternating rows of stationary airfoils 18 (commonly referred to as vanes) and rotating airfoils 20 (commonly referred to as blades). Each row of blades 20 is formed by a plurality of airfoils 20 attached to a disc 22 provided on a rotor 24. The blades 20 can extend radially outward from the discs 22 and terminate in a region known as the blade tip 26. Each row of vanes 18 is formed by attaching a plurality of vanes 18 to a vane carrier 28. The vanes 18 can extend radially inward from the inner peripheral surface 30 of the vane carrier 28. The vane carrier 28 is attached to an outer casing 32, which encloses the turbine section 16 of the engine 10.
Between the rows of vanes 18, a ring seal 34 can be attached to the inner peripheral surface 30 of the vane carrier 28. The ring seal 34 is a stationary component that acts as a hot gas path guide between the rows of vanes 18 at the locations of the rotating blades 20. The ring seal 34 is commonly formed by a plurality of metal ring segments. The ring segments can be attached either directly to the vane carrier 28 or indirectly such as by attaching to metal isolation rings (not shown) that attach to the vane carrier 28. Each ring seal 34 can substantially surround a row of blades 20 such that the tips 26 of the rotating blades 20 are in close proximity to the ring seal 34.
During engine operation, high temperature, high velocity gases flow through the rows of vanes 18 and blades 20 in the turbine section 16. The ring seals 34 are exposed to these gases as well. Some metal ring seals 34 must be cooled in order to withstand the high temperature. In many engine designs, demands to improve engine performance have been met in part by increasing engine firing temperatures. Consequently, the ring seals 34 require even greater cooling to keep the temperature of the ring seals 34 within the critical metal temperature limit. In the past, the ring seals 34 have been coated with thermal barrier coatings to minimize the amount of cooling required. However, even with a thermal barrier coating, the ring seal 34 must still be actively cooled to prevent the ring seal 34 from overheating and burning up. Such active cooling systems are usually complicated and costly. Further, the use of greater amounts of air to cool the ring seals 34 detracts from the use of air for other purposes in the engine.
As an alternative, the ring seals 34 could be made of ceramic matrix composites (CMC), which have higher temperature capabilities than metal alloys. By utilizing such materials, cooling air can be reduced, which has a direct impact on engine performance, emissions control and operating economics. However, there are a number of natural limitations and manufacturing constraints associated with CMC materials. For instance, laminated CMC materials (oxide and non-oxide based) can have anisotropic strength properties. The interlaminar tensile strength (the “through thickness” tensile strength) of the CMC can be substantially less than the in-plane strength. In addition, anisotropic shrinkage of the matrix and the fibers can result in de-lamination defects, particularly in small radius corners and tightly-curved sections, which can further reduce the interlaminar tensile strength of the material.
Ceramic matrix composite ring segments would typically be attached to the metal backing hardware away from the gas path where temperatures are more favorable for metals. However, as a result of such an arrangement, some of the CMC features are situated out of plane; that is, the fibers of the CMC material are not parallel to the surface of the component exposed to the hot gas path. Such out of plane features include, but are not limited to, flanges, hooks, T-joints, etc. During engine operation, differential pressure loads and other mechanical loads must be reacted by these out-of-plane features with the load path through a transition region between the features and the hot gas path surface. For instance, some ring seal segments are cooled by supplying a pressurized coolant to the backside (or “cold” side) of the ring seal segment. The coolant is at a greater pressure than the hot gases flowing through the turbine section to prevent the hot gas from being ingested in this area. As a result, the ring seal segment is subjected to pressure loading, which must be transmitted to the attachment points of the CMC ring seal segment. However, in order to do so, the pressure loading must be transmitted to the attachment points on the out of plane CMC features through a transition region (such as a fillet or other transition region) where the material is weakest. Such areas tend to be design-limiting features of these components.
Thus, there is a need for a CMC ring seal segment construction that can minimize the limiting aspects of CMC material properties and manufacturing constraints and improve the mechanical loading capability.